Conventionally, magnetic compasses are used to sense terrestrial magnetism and provide a visual indication identifying at least one predetermined compass heading such as north, south, east or west. Recently, magnetic flux gate compasses have been utilized to sense terrestrial magnetism and to provide an electronic signal which identifies the predetermined compass direction. Because these devices operate in response to sensed terrestrial magnetism, they are somewhat inaccurate since they are subject to local disturbances and variations of the earth's magnetic field. In addition, the magnetic north sensed by such sensors does not, in general, correspond to the true north compass direction and therefore correction or calibration of the compass output may be required if accurate compass headings are to be achieved.
The electronic direction finder described in the above noted copending U.S. patent application utilizes a flux gate compass to provide an electronic signal indicative of the orientation of a global positioning system (GPS) receiver, and its visual display, with respect to a predetermined compass heading such as north. A signal is also generated which determines, which respect to predetermined compass directions, the direction, from the receiver, of a desired destination. The prior direction finder then provides on its display a rotatable pointer which provides a visual indication of the bearing towards a selected desired destination. The sensing of terrestrial magnetism by the flux gate compass determines the orientation of the GPS receiver with respect to compass directions and ensures the proper orientation of the displayed visual arrow to the desired destination.
The electronic direction finder described in the above identified copending U.S. patent application represents a substantial improvement over other direction finders currently available. It is contemplated that such a direction finder will be of a size so as to be hand holdable and therefore represent a portable navigation device usable by hikers and other pedestrians as well as usable by boaters. However, terrestrial magnetism sensors such as flux gate compasses are relatively expensive and their cost therefore might prevent the widespread adoption and commercial success of such a hand held GPS receiver. Also, the flux gate compass and its associated circuitry appreciably adds to the total size of a hand held GPS receiver when included therewith. In addition, sensing of terrestrial magnetism would also require calibration to correct for known variations of the earth's magnetic field. Also, such a system would be susceptible to errors in sensed magnetic field cause by local disturbances of the earth's magnetic field.
Thus there is a need for an electronic direction finder apparatus which is relatively inexpensive and small in size and does not utilize a flux gate compass or any sensing of terrestrial magnetism in order to produce an accurate visual display of compass directions and/or the direction to a desired destination.
Some prior direction systems rigidly mount two widely spaced omnidirectional antennas on a vehicle, such as a ship, and calculate the direction of the vehicle heading by calculating the position of each fixed antenna location. This is not practical for a small hand held direction finder device.